The present invention relates to a modem pooling system for enabling selective connection of a plurality of data terminal units to a modem through exchange service.
A modem is mainly used to connect a data terminal unit to an analog line such as a public telephone line. The modem pooling system is a system for enabling a number of data terminal units to get access to an analog line by use of a few modem, and practically realized in a digital key telephone system, for instance.
In the digital key telephone system of this type, conventionally, at least one modem is connected to a key service unit including an exchange circuit via an interface (e.g. RS-232C), and further an analog office line is connected to the modem. Further, a plurality of extension key telephone sets connected to the key service unit are provided with each interface unit (e.g. RS-232C) respectively, through which each personal computer is connected, as a data terminal unit, to each extension key telephone set.
In the conventional system, when an incoming call arrives at the modem through the office line, the incoming call signal is first transmitted from the modem to a control unit in the key service unit. In response to this signal, the control unit controls a switching circuit for exchange service to connect the modem to a data terminal unit, and further indicates the incoming call arrival to the data terminal unit. In response to the incoming call information, the data terminal unit starts data communication with the office line.
In the conventional system, however, where an automatic responsive modem is used, there exists a problem in that when the control unit is getting access to the data terminal unit to be called, the modem responds to an incoming call signal, so that before the modem is connected to the data terminal unit, data are transmitted from the office line to the modem and therefore the head portion of the transmitted data cannot be received by the data terminal unit.
In more detail, the automatic responsive modem usually responds automatically to an incoming call signal arrival by an independent control unit incorporated in the modem, after a predetermined time has elapsed, irrespective of the operation of the key service unit. However, when the control unit in the key service unit is under overload conditions (the state where a plurality of outgoing or incoming call signals are generated simultaneously and therefore each control processing is delayed momentarily), since the exchange service is delayed from the automatic response of the modem, the head portion of the data inputted from the modem to the key service unit will not be transmitted to the data terminal unit.